1. Technical Field
The present invention relates to semiconductor fabrication, and more particularly to devices and methods with a retrograde doping well to reduce or eliminate punchthrough effects in finFETs formed in bulk substrates.
2. Description of the Related Art
Punchthrough effects are parasitic leakage currents that occur in metal oxide semiconductor field effect transistors (MOSFETs) between source and drain regions. Leakage current of a metal oxide semiconductor (MOS) transistor may increase due to a parasitic current path that exists between drain and source regions. Such a parasitic path exists in bulk MOSFETs, for example, in a region that is between the source and drain but deeper below a gate. This component of the drain current is poorly controlled by the gate since the current path is located in the bulk farther away from the gate. This parasitic current adds to sub-threshold leakage current resulting in increased power consumption.
A fin field effect transistor (finFET) constructed on a silicon-on-insulator (SOI) substrate has an advantage that punchthrough effects are substantially reduced due to the presence of a buried oxide that minimizes drain-to-source coupling at the fin bottom. In bulk substrates, however, the punchthrough effect is significant in the region below the fin. One way to reduce the punchthrough effect is to increase the overall bulk substrate doping level. As a result the drain and source depletion regions will become smaller and will not establish a parasitic current path. This substrate doping however can diffuse up the fin during subsequent processing resulting in a higher threshold voltage at the bottom of the fin compared to the top. To offset this, the fin may have to be tapered (wider at bottom compared to the top) to maintain approximately uniform threshold throughout the fin height. Further, random dopant diffusion up the fin can result in threshold voltage (Vt) variations. A better alternative is needed to prevent punchthrough, maintain uniform threshold throughout fin height and mitigate Vt fluctuations.